Paperboard closures which are used for cylindrical food containers, such as cylindrical salt boxes, are frequently made of a material known as cap stock. Conventionally, such cap stock is formed from flat paperboard by first making circular cutouts of the paperboard and then drawing such cutouts into shallow cup-shaped caps. Glue is applied to the portions of the cylindrical containers which are overlapped with the caps when assembled. After application of the glue, the caps are placed over the container open end, or ends, and the overlapping portions are pressed together and held in such position until a suitable bond is formed by the applied glue. Usually, the cutting, drawing, glueing, positioning and closing steps take place in an assembly-type operation, in a continuous manner.
The conventional cap stock which is in use today employs recycled board as a substrate. The outer surface of the substrate is covered with a bleached white liner board adhered to the substrate by a suitable adhesive. The opposite inner side of the substrate is covered with a virgin brown liner board adhered to the substrate by a polyethylene adhesive. The polyethylene adhesive, in addition to having adhesive properties, also functions as a moisture barrier to the transmission of moisture from the outside to the ingredients within the container, for instance salt. The use of virgin brown liner board on the substrate inner side is necessary because of contact of the cap with food items.
In drawing the cap stock, the circular cutouts, which may be about 4" in diameter, are provided with a rim of about 3/8" in height. It is this rim which overlaps a part of the container sides when the drawn cutouts are seated on a container open end. Thus, glue must be applied to an exposed container end for about the same distance as the rim height (about 3/8").
A problem with this conventional process is the need for the use of glue, the accompanying glue pots, and attendant difficulty in cleaning equipment and glue pots after shutdown or between runs. A glue-free procedure would have obvious advantages.
It has been proposed to reduce the cost of the cap stock by replacing the laminate with a recycled board coated with a moisture barrier coat, such as a polyvinylidene chloride coat. While this may reduce the cost of the cap stock, it still requires the step of glueing the cap to the container, in turn incurring the attendant costs associated with glueing, such as cleaning of glue pots.
U.S. Pat. No. 2,586,446 to Stockburger, describes a closure said to be particularly useful for glass bottles. The closures, which are formed from a plurality of layers of cellulosic material, are adhered to the bottles or other containers by means of a plastic cement such as vinyl acetate, vinyl chloride copolymer, polyethylene, polyvinyl chloride and the like. The plastic cement bond is achieved by a combination of heat and pressure applied by means of a forming die used to form the cap or closure over the container open end. The plastic cement is said to have a greater adherence to the laminate than it does to the receptical, and in this respect it is emphasized in the patent that the bond that it forms with the receptical is readily breakable by simply hand twisting the closure.
The closure is said to be gas-impervious and also water tight, although it is indicated in the patent that the closure may permit gradual passage therethrough of water vapor. It is suggested that some plastic cements may have water vapor imperviousness.
In the method of this patent, a roll of closure film is fed to a capper head wherein it is cut into short lengths or segments which are then positioned between the capper head and a bottle to be closed. The capper head is heated to a temperature above the temperature at which the plastic cement on the film is adherent to the bottle. The head has a recess which presses the center of the severed piece of film firmly onto the top of the bottle and presses the skirt of the closure firmly against the sides of the bottle. The combination of the heat and pressure forms a tight seal between the closure and bottle. The disclosure of this patent is incorporated by reference herein.
A Crown-Zellerbach U.S. Pat. No. 3,259,507 describes a heat-sealable seal for food packages comprising a laminate which is composed of a base sheet, an intermediate release or delaminating layer, and an inner adhesive layer, which is said to be heat activatable. The purpose of the release layer is to permit the base sheet, which seals the package, to be removed when access to the package is desired. There is no reference in the patent to the presence of a release coat on the base sheet opposite to side of the adhesive coat, or to moisture resistant properties of the adhesive coat.
It is well known to coat a substrate on one side with an adhesive and the other side with a release coat. This is shown, by way of example, in Johnson U.S. Pat. No. 4,202,925, directed to a tape, and in Avery International U.S. Pat. No. 4,050,121, directed to a tab. The subject matters of these patents are not closures in the same sense as in the present invention.
Other patents illustrating the prior art include a Kraft Foods U.S. Pat. No. 2,649,392 on a cover of chlorinated rubber, heat sealable to a container of vinyl chloride acetate. The cover is said to be readily peelable from the container. An Alton Boxboard Co. U.S. Pat. No. 4,075,372 describes the use of ethylene vinyl chloride as a flexible precoat on boxboard and paperboard to alleviate cracking and fracturing along a scored or folding line. The precoat is top coated with a ladquer having moisture barrier properties. A Phillips Petroleum Co. U.S. Pat. No. 4,258,529 describes a cover for containers comprising a metal foil substrate and top and bottom thermoplastic coatings. Also, Metal Box Limited U.S. Pat. No. 4,362,002 describes a laminate strip for making closures comprising a paper substrate and a coating of a polymer of ethylene and acrylic acid. There is no reference in this patent to release or slip properties of the laminate strip.
Patents on heat seals for use in packaging include W. R. Grace & Co. U.S. Pat. No. 4,352,702; Tamarin U.S. Pat. No. 2,845,213; Smith U.S. Pat. No. 2,984,573; and Berst U.S. Pat. No. 2,829,791. The Smith patent is directed to art related to that of the Crown-Zellerbach U.S. Pat. No. 3,259,507, mentioned above.